The present invention relates generally to solar furnaces and more particularly to one particularly suited to heating a transient fluid flow. Solar furnaces as presently known, incorporate complex lens or reflector arrangements for converging solar rays. Such arrangements usually include a battery of lenses mounted in a movable manner by complex assemblies necessary to accomplish continuous convergence of light rays toward some type of heat exchanger. Lenses used are of biconvex nature and hence of substantial cost which, along with their complex support assemblies, renders the solar furnace impractical for use other than for very large size installations or experimental uses.